This application is based on Patent Application No. 11-040704 filed in Japan, the contents of which are hereby incorporated by reference.
This invention relates generally to a light-pickup device applied to an optical disk device for recording, reproducing, and erasing information in an optical disk having plural data recording layers. Particularly, the invention relates to detection of timing for a changeover between acceleration and deceleration of focus drive voltage when the light-pickup device changes between information recording layers.
Conventionally, an optical disk device which records, reproduces or erases information in an optical disk laving multiple information recording layers, such as a single-side two-layer DVD, needs to read out the information which is recorded in each of Layer 0 and Layer 1 from one side of the optical disk as shown in FIG. 1. Therefore, in order to change from a state in which a light-pickup device is reading out information recorded in Layer 0 into a state in which it reads out information recorded in Layer 1, the optical disk device performs an operation that is known as xe2x80x9cfocus kickxe2x80x9d. The focus kick is an operation for accelerating or decelerating focus drive voltage applied to an actuator which drives a tens of the light-pickup device. Controlling timing for carrying out the focus kick enables movement of the lens of the light-pickup device from a position for performing the focus servo in one layer to a position for performing the focus servo in the other layer.
That is, the acceleration kick of the focus drive voltage is performed in the state that the focus servo is carried out for Layer 0, and then the deceleration kick of the focus drive voltage is performed, which makes it possible to change over into a state for performing the focus servo for Layer 1 from for Layer 0. In this case, the timing for the deceleration kick of the focus drive voltage has to be specified. Generally, a point (referred to as focus zero cross point) where output value from a focus error signal crosses a focus zero cross value (predetermined reference value) is employed for the timing. However, as shown in FIG. 2, in an optical disk having multiple information recording layers, when the focus drive voltage is gradually increased, an output value of the focus error signal forms a curve having two S-shapes in response to a change in a distance between a lens and an image recording layer of the disk with the lens movement. This makes a zone in which no focus error signal is occurred at midpoint of a changeover of focusing from one layer to the other, so that the focus zero cross point cannot be detected in this zone, which makes it unable to specify a point of kicking the drive voltage for deceleration. Therefore, as shown in FIG. 4, the light-pickup device has a constitution in which a switch mechanism 43 is provided for applying an offset to the focus zero cross point detection circuit 41 in the servo-processor (IC (integrated circuit)) 4, thereby applying the offset voltage at a reference value level of a comparator for detecting the focus zero cross. Owing to said constitution, as shown in FIG. 3, the focus zero cross (FZC) point can be detected in a state so as to make no coincidence between the zone in which no focus error signal is occurred and the focus zero cross. The control for changeover between the acceleration and deceleration of the focus drive voltage is carried out by using the detected focus zero cross point.
However, the above-described conventional light-pickup device requires a servo-processor which enables change of the reference value level of the comparator for detecting the focus zero cross. Also, such a servo-processor needs an external circuit, which complicates the construction of the circuit her, as shown in Japanese Patent Laid-Open publication 9-270136, there has been provided a light-pickup device which has a function for preventing a malfunction caused by a peak point due to spherical aberration of an optical component in an S-shaped curve drawn by output value of the focus error signal. However, even this device cannot resolve the above-mentioned problems.
This invention is made to solve the above-mentioned problems. One object of the present invention is to provide a light-pickup device which is capable of detecting a focus zero cross point of a focus error signal with a simple circuit configuration even if the device still employs a servo-processor which is unable to change a reference value level of the comparator for detecting the focus zero cross.
In order to achieve the above-mentioned object, according to one aspect of the present invention, a light-pickup device for an optical disk device which performs recording, reproduction or erasing of information of an optical disk having, plural information recording layers comprises: a light source; a lens for gathering light flux generated by said light source onto an image recording plane; a lens driver for driving said lens; a focus drive voltage controller for controlling focus drive voltage applied to said lens driver, which kicks focus drive voltage for acceleration when the information recording layer is changed over, and kicks the focus drive voltage for deceleration when a focus zero cross is detected from a focus error signal detected based on reflected light flux from the information recording plane; and, an offset voltage controller which applies offset voltage to the focus error signal when the focus drive voltage controller is activated for changing over the information recording layer.
In the above-described constitution, the offset voltage controller applies the offset voltage to the focus error signal, thereby, the focus zero cross point can be detected in a state so as to make no coincidence between the zone in which no focus error signal is occurred and the focus zero cross. The control for changeover between acceleration and deceleration of the focus drive voltage is carried out by using the detected focus zero cross point. Also, the focus zero cross point can be detected only by placing a simple structure for applying the offset voltage to an amplifier of the focus error signal without giving any changes to an IC (integrated circuit) such as a servo-processor comprised in the focus drive voltage controller of said light-pickup device.
According to another aspect of the present invention, the offset voltage controller applies offset voltage to the focus error signal when the focus drive voltage controller kicks the focus drive voltage for acceleration, and finishes applying the offset voltage to the focus error signal when the focus drive voltage controller kicks the focus drive voltage for deceleration.
In the above-described constitution, the offset voltage controller applies the offset voltage to the focus error signal only during time required for detecting timing of kicking the focus drive voltage for deceleration, and applies no offset voltage to the focus error signal after a termination of the deceleration kick. Accordingly, an accurate detection can be given whether the focus servo is performed in an information recording layer which is changed over.
According to a further aspect of the present invention, the offset voltage controller comprises: an operational amplifier having one input terminal to which a focus error signal is input, and another input terminal to which offset voltage is applied; a circuit means for changing over said offset voltage in response to a normal playing mode and a mode for changing the information recording layer; and, a controller for executing control for changing over said circuit means.
In the above-described constitution, the controller executes control for changing over connection of the circuit means having one input terminal to which a focus error signal is input, and another input terminal to which offset voltage is applied, thereby executing control for applying the offset voltage to the focus error signal. Accordingly, the offset voltage can be applied to the focus error signal only by adding (also, possible to built in) a simple circuit to an operational amplifier of the focus signal without placing an external circuit in the IC such as a servo-processor.